Construction issues relating to shallow deck composite floors
Because composite floors have a maximum span of about 3.5m, the supporting structure is generally based on a system of secondary beams at corresponding centres.
Composite floors are invariably used in conjunction with steel beams for the supporting structure. Because of the problems of deflection and strength during construction, greatest efficiency is gained when the slabs span a relatively short distance. Typically this may be about three metres depending upon the particular profile and design conditions. This does not imply such a closely spaced grid of columns. Instead a system of secondary beams is usually adopted. These may span up to ten metres or more, enabling large column-free spaces to be achieved.
The decking is simply fixed to the steel beams and provides lateral restraint.
The decking can be fixed to the supporting steel beams using shot-fired pins, self-tapping screws or welding. Lateral restraint to the steel framed structure can be achieved by ensuring that sufficient fixings are used. Where composite construction is required shear studs are normally welded through the deck as described earlier.
Lightweight concrete reduces loads but may cause some construction difficulties.
In order to reduce loads both during construction and in service, use is often made of lightweight concrete. This may have a unit weight of as little as 15 kN/m3 compared with that for normal weight concrete of 24 kN/m3. Use of very lightweight concrete can result in difficulties in fixing mesh reinforcement in the top of the slab, and typically an intermediate weight (19 kN/m3) will be used.
Conventional reinforcement is often placed in the concrete over the supporting beams and more generally to resist cracking.
Where the slab crosses supporting beams the bending moments and hence the bending stresses become reversed. Under these circumstances the concrete becomes stressed in tension and it is therefore necessary to provide conventional reinforcement to accommodate this.
As concrete cures there is a natural tendency for it to shrink. To avoid cracks developing on the surface of the slab due to this shrinkage, anti-crack reinforcement, in the form of a relatively light mesh of steel bars, should be provided.
Composite floors generally perform well in fire but may require additional light reinforcement for long periods of fire resistance.
The composite floor must provide satisfactory performance in terms of stability, integrity and insulation in the event of fire. The last of these is simply dependent upon the thickness of the slab, whilst the integrity is generally ensured by the sheeting. This acts as a shield to the concrete, and helps to contain and control spalling. The stability of the structure, that is its ability to avoid collapse during a fire, is dependent upon conventional reinforcement in the slab. It may be that the nominal anti-crack reinforcement is adequate for this purpose, but a small amount of additional reinforcement may be required, particularly in the bottom of the slab, to satisfy this condition.
Longer span slabs can be achieved using temporary props, but this interferes with subsequent construction activities.
In order to increase the span of a composite floor use can be made of temporary propping until composite action has been achieved. The steel sheeting is typically supported at its mid-span, thereby considerably reducing both bending stresses and deflections at this stage of construction. This does however eliminate one of the principal advantages of using composite floors, namely the speed of construction resulting from a free working space on lower floors. As a result unpropped construction is normally used with closely spaced secondary beams.
